Porto will face a disciplinary hearing after opponents Manchester City complained that striker Mario Balotelli suffered racist abuse during their Europa League match last Thursday.
European football's governing body UEFA said it was investigating "improper conduct" by Porto supporters and the hearing would take place on March 29.
"Following a complaint lodged by Manchester City, UEFA has today opened disciplinary proceedings against Porto for the improper conduct of supporters," UEFA said in a statement.
Racism is a hot topic in English football at present with Liverpool's Luis Suarez having served an eight-match ban for racially abusing Manchester United's Patrice Evra.
Chelsea captain John Terry is awaiting trial after pleading not guilty to a charge of racially abusing Queens Park Rangers defender Anton Ferdinand during a match last October.